A laser gyroscope or laser gyro, is an optical movement sensor for measuring the rotation speed of the reference frame of the sensor relative to a Galilean reference frame about one or more axes. Laser gyroscopes are sensors used in inertial measurement units or IMUS. These IMUS are found in inertial control units or sensor units of inertial navigation systems on board certain types of vehicle, such as aircraft.
A laser gyroscope only comprises a one-piece optical block having one or more laser cavities, one cavity per measurement axis, each optical cavity being formed by a polygonal assembly of mirrors on the optical block and a generally gaseous amplifying medium. It also comprises a mechanical activation structure to which the optical block is fixed, said structure serving to generate an alternating rotational movement of the optical block about what is called an activation axis. According to the prior art, this mechanical structure uses an activation wheel comprising an outer part, or rim, a hub in the form of a cylinder of revolution, plane radial plates between the rim and the hub, and a one-part piece called a “tulip” that forms a mechanical link between the optical block and the hub of the activation wheel.
This mechanical structure must provide a mechanical link as rigid as possible with respect to tilting and translational vibration modes of the optical block on the wheel.
To meet this requirement for rigidly fixing the optical block, one technical solution of the prior art is to use two activation wheels that are fixed either side of the optical block, relative to the activation axis, by an associated tulip, and more precisely a thick wheel in the direction of the activation axis but which gives the translational rigidity in this direction, and a thinner wheel in this same direction that prevents the optical block from tilting in a direction perpendicular to the activation axis while still allowing differential expansions to take place reversibly between the internal and external bearing surfaces of the wheels. This technical solution is used notably in the case in which the optical block comprises three measurement cavities (triaxial gyroscope block), taking into account the masses and inertias then involved
This mechanical structure must also prevent any relative slip between parts over the temperature range in which the gyroscope is stored and operated, which in the avionic context may extend from −55° C. to +100° C. The operation of a laser gyro optical block is accompanied in fact by a temperature rise of the optical block of around ten to twenty-five degrees above the ambient temperature, whereas the environmental temperature to be taken into consideration in the thermal design of a piece of equipment using a laser gyro typically extends over a range from −15° C. to +70° C. Moreover, in storage, this kind of equipment may undergo larger variations, with temperatures dropping down to −45° C. or even −52° C.
According to the prior art, and taking into account the extent of the temperature range for operating and storing the gyroscope, the parts of the mechanical activation structure (wheels and tulips) are generally made of a material having a very low thermal expansion coefficient over this temperature range for the purpose of minimizing the linking forces due to expansion differentials.
Moreover, it is known that a laser gyro optical block is normally machined from a ceramic having a very low expansion coefficient (typically of the order of 10−2 ppm/° C.), such as Zerodur™, so as to limit the dimensional variations of the cavity or cavities in optical resonance during its operation. The parts of the mechanical activation structure, which must accommodate the loads due to inertial forces (coming from the activation of the optical block and from the shocks and vibrations of the gyroscope support vehicle) as rigidly as possible, so as to minimize the spurious movements of the optical block, without generating excessive stresses on the optical block during the abovementioned temperature excursions, are therefore advantageously made of a metal alloy having a low expansion, such as Invar™, which is an alloy containing 36% nickel and 64% iron.
Thus, many precision laser gyros used on civil or military aircraft, for purely inertial location purposes, the wheels and the tulips fixed to the optical block (by adhesive bonding or by crimping) are made of Invar™, the expansion coefficient of which is remarkably low over the temperature range to be taken into account.
The mechanical linkage between the wheel hub and the tulip is then advantageously a screw-clamped plane-to-plane linkage, the tangential loads under the screw heads or between the parts, caused by the thermal expansion of these parts, being limited by the homogeneity of the materials. This clamped mechanical linkage has the advantage of enabling the assembly to be easily mounted and removed, thereby facilitating both industrial production and repair. More precisely, the tulip comprises a cylindrical mount, having a plane face which is clamped by screws onto the hub of the activation wheel, which mount is also cylindrical, the other face of the tulip comprising fixing tabs for bonding the optical block.
Although this technical solution is very satisfactory as regards the aspects of mechanical rigidity, reduction in stresses due to expansion differentials and ease of mounting and removal, it does have a major drawback, namely its cost, due to the cost of the material itself—Invar™—and due to the difficulty of machining it.
For less demanding applications in terms of metrological precision of the sensor, other mechanical assemblies have been used that comprise steel wheels between which the optical block is fixed by pinching. However, such a technical solution can be applied only if a lower level of overall precision is accepted, on account of the risks of mechanical instabilities at temperature.